The ultrasonic propagation properties of living systems, and preparations of their molecular constituents, are studied with the views toward insuring more effective clinical use of this modality and toward obtaining a more profound understanding of the physical interactions involved. Special attention is devoted to the absorption properties and mechanisms. Recent findings suggest that mammalian gonads may exhibit rather unique properties, vis-a-vis other organs, and that structural proteins may provide for greater absorption than do the structural proteins. Continued development of appropriate instrumentation is also conducted. BIBLIOGRAPHIC REFERENCES: F. Dunn, Acoustic Properties of Biological Materials, in Fundamental and Applied Aspects of Nonionizing Radiation, ed. by Michaelson, Miller, Magin and Carstensen, (Plenum Publishing Co., New York, 1975). pp. 21-40.